1. Field of the Invention
The present invention relates to gas generators for inflating vehicle inflatable restraint cushions, commonly known as air bags, so as to provide impact protection to occupants of the passenger vehicles. More particularly, this invention relates to an improvement in providing varying rates of inflation of the air bag dependent on crash conditions and/or occupant conditions.
2. Description of Related Art
An air bag restraint system, referred to as a module, typically includes a canister which encloses a gas generator or as, it is commonly known, an inflator, and at least part of an air bag, and a cover which conceals the module from view. When the vehicle is involved in a collision, a crash signal initiates operation of the inflator to cause the air bag to deploy. The inflator produces an inert gas (e.g., nitrogen) which is directed under pressure into the air bag to force the air bag out of the canister incorporated into the module and into the passenger compartment of the vehicle. In a pyrotechnic type inflator, gas is produced by the burning of a gas generating material. As the air bag is forced out of the container, pressure exerted on the cover causes selected portions of the cover to separate in a predetermined manner along tear seams to enable the air bag to be directed into the passenger compartment. As the air bag is directed into the passenger compartment, it is inflated by-the continued flow of gas produced by the inflator.
With a pyrotechnic type inflator, the rate at which the gas generant must burn in order to provide gas at a sufficient rate to inflate the air bag, so as to protect the occupant, is dependent on a number of factors; i.e., the speed at which the vehicle is moving, the severity of the crash, the size and location of the occupant and the temperature of the inflator. For example, when a vehicle moving at a moderate rate of speed (30 miles per hour/50 kph) with an inflator at a temperature of approximately 70.degree. F. (21.degree. C.) is involved in a collision, a "normal" rate of gas production from the inflator must be sufficient to deploy the air bag within the time required to protect the occupants. When a vehicle moving at a high rate of speed (50 mph/85 kph) impacts another vehicle moving at approximately the same speed, a higher rate of gas production from the inflator may be required to provide the same level of protection to the occupants.
An additional concern is the capability to compensate for the reduced reaction rate of cold gas generant by increasing the gas production rate of the gas generant. When a vehicle moving at a high rate of speed (50 mph/85 kph) with an inflator that is cold, due to being maintained at sub-zero temperatures -20.degree. F. (-29.degree. C.) collides with a stationary object, it may be necessary to compensate for the reduced rate of gas production caused by the low temperature in order to deploy the air bag in sufficient time to protect the occupants.
In addition, in order to provide a less sudden and softer action of the air bag cushion more closely tailored to the size and weight of the occupant so as to reduce the chance of possible injury to the occupant because of air bag cushion deployment, it is desired that the initial inflation of the air bag cushion be more gradual but with the same or only a slightly longer time required for total inflation. There is a greater need for this gradual inflation when a small child is present on the passenger side.
Prior art solutions to these problems have been accomplished by two-stage, or dual performance inflators which use separate cylinders, bulkheads, and alternated ignition trains incorporated in inflators of this type. Such prior art solutions are disclosed in Kirchoff et al., U.S. Pat. No. 3,972,545; Schneiter el al., U.S. Pat. No. 4,358,998; and Blanchard, U.S. Pat. No. 3,713,667.
Kirchoff et al., U.S. Pat. No. 3,972,545, which invention is assigned to the assignee of the present invention, is incorporated herein by reference. Kirchoff discloses a consumable partition in a gas generator divided into two chambers each chamber containing gas generant material and being ignitable by a separate igniter. An impact sensor determines whether one or both of the squibs are fired on impact, depending upon the force of a collision, and hence, the rate at which the gas bag is filled with gas.
Schneiter el al., U.S. Pat. No. 4,358,998, which invention is assigned to the assignee of the present invention, discloses an igniter which causes the gas generating material in the inflator to ignite in a progressive manner so as to inflate the bag slowly at the onset but more rapidly later as inflation progresses. The igniter has a disc delay between two portions thereof whereby the gas generant material around one portion is ignited immediately and then after a delay, the gas generating material around the other portion is ignited.
Blanchard discloses a pressurized container for providing gas to an air bag cushion, where the passage communicating with the air bag has multiple orifices. Each orifice is closed to the pressure vessel by a diaphragm rupturable by electrically fired detonators. The number of orifices to be opened is dependent on the force of a collision and the rate at which the air bag is to be filled with gas. A similar solution to this problem has been suggested by Vos, U.S. Pat. No. 3,741,580, which discloses a multiple level flow rate of a pressurized fluid to a cushion by selective operation of serially disposed flow control orifices in accordance with predetermined levels of intensity of impact.